You've probably given upon getting an answer by now but here goes.

Firstly, I think it's not totally accurate to say that the popularity of the mentalité/microhistory represented a movement away from the Annales of Braudel, Bloch, and Febvre. The movement had already moved away from their histoire totale and started pursuing the histoire problème, with its emphasis on quantification and use of computerized data.

The mentalités, then, marked a new shift in the direction of the Annales. This placed an emphasis on the life experiences of ordinary people -- rather than what was criticised as the "geographic (or ecological) determinism" of earlier Annales scholars -- seeking to shine a light on the inner workings of society. In a sense this emphasis on the historical agency of people was a clean break from the deterministic approaches of both previous generations of Annalistes. In another, however, it rescued Braudel and co's longue durée (which had been neglected by the quantitavists) but with the addition of a new dimension, that of mental structures (hence the name mentalités, meaning mentalities).

So why did they narrow their focus? Because they wanted to take that new dimension of mental structures/mentalities into account. Apart from Ladurie, good examples of this are Ginzburg and Darnton.

I hope this helped answer your question! I'd be glad to cite some sources, suggest further reading, or answer any follow-up questions if asked to.